“Because of Nikki Haley’s leadership, the United States has a powerful voice again at the United Nations,” he said.

Gardner, a member of the Senate Foreign Relations Committee, said Haley helped the administration apply its “maximum pressure doctrine on North Korea.”

Haley was also one of the few members of Trump’s Cabinet to stand up to him regularly.

In an op-ed last month, Haley said she never hesitated to air her differences of opinion directly with Trump.

“I don't agree with the president on everything. When there is disagreement, there is a right way and a wrong way to address it. I pick up the phone and call him or meet with him in person,” she wrote.

Her editorial was in response to the anonymous op-ed by a “senior official” in the Trump administration that appeared in The New Times on Sept. 5. It described a clandestine effort in the Cabinet to rein in the president.

Some observers thought that Haley might have been the secret author.

Speaking with Trump from the Oval Office on Tuesday, the former governor praised the president's leadership and denied having any intention challenging him in the next White House election cycle.

"No, I am not running in 2020," she said, adding, “I look forward to supporting the president in the next election.”